Links across interconnected networks vary substantially with respect to such factors as bandwidth, packet latency, error and loss rates, and network medium (asymmetric or symmetric broadband, analog dialup, wireless networks, etc.). Before network protocols and distributed applications are deployed in a real network, it is critical that they be thoroughly tested under various realistic network conditions, to ensure correctness and to verify performance characteristics. Testing in an isolated real network is generally impractical. Therefore, testing typically involves simulation and emulation.
Network simulators generally attempt to provide a rich set of protocol modules and configuration tools suitable for conducting customized simulation experiments. However, network simulation suffers from a number of limitations. Simulators rely principally on models of both the physical network infrastructure and networking protocols. Models by definition only approximate the real conditions being tested, and in complex situations it is often impossible to develop accurate models for purposes of simulation. The functionalities provided by simulation modules are merely logical operations; thus, an implementation in a simulator must be modified before it can be deployed within a target network. Network simulators consume significant resources when the network being simulated is sufficiently large, and they do not provide a view of the network end user's experience.
By contrast, network emulators permit applications and protocols to be tested in real time, on real machines, such as locally-linked computers, using real implementations of network protocols. An emulator includes a supplementary means for imposing synthetic delays and faults on the real network traffic. In effect, the emulator comprises a virtual network with respect to the host machine or machines on which the network applications being tested are running. For a network emulator to be useful, however, it is necessary that it be designed and structured in such a way that various large-scale network conditions may be emulated accurately and realistically as well as efficiently and economically.